1. Field of Invention
The present invention relates to a process for removal of contaminants less dense than water from pulp and paper mill liquids, and, more especially, to a process for deresination of pulp and paper industry process liquids.
2. Description of Prior Art
A small fraction, (usually 1-5%) of the weight of wood from all species of trees consists of wood resin (Mutton, D. B., "Wood Extractives and Their Significance to the Pulp and Paper Industries". Ch10, Wood Resins, Ed. W. E. Hillis, Academic Press, New York, (1962)). Wood resin is usually defined as the material in wood which is insoluble in water, but soluble in organic solvents (Mutton, D. B., "Wood Extractives and Their Significance to the Pulp and Paper Industries". Ch. 10, Wood Resins, Ed. W. E. Hillis, Academic Press, New York, (1962)). Resin is present in wood mainly in two locations: the parenchyma cells and the resin canals (Back, E. Svensk Papperstidn. 63(22): 793 (1960); Back, E., Svensk Papperstidn., 72(4): 109 (1969)).
When wood is pulped, be it by a chemical process, a mechanical process, or a combination of the two, the wood resin remains in a number of physical locations (L. H. Allen, Pulp Paper Can., 76(5):70 (1975)): on the surfaces of fibers (in patches and droplets), inside parenchyma cells, on the surfaces of pulp fines, dissolved in the pulp white water (at pH values above 6), and dispersed in the pulp white water (L. H. Allen, Trans. Tech. Sect. CPPA, 3(2): 32 (1977)).
In a chemical pulp mill, wood chips are cooked in the presence of certain chemicals to dissolve the lignin between the fibers. At the end of this process the resulting pulp is washed to remove residual cooking liquor containing dissolved and dispersed lignin. In this process a certain amount of dissolved and dispersed resin is also removed. However, industrial washing is seldom complete and a substantial fraction of the wood resin which is dissolved and dispersed in the process liquid of the wet pulp usually remains and continues downstream. The pulp is subsequently bleached with various chemicals and at the end of each chemical treatment it is usually washed. Ideally, the wash water from each washer is discarded; nevertheless, because of environmental pressures to reduce effluents from mills, this is rarely done and the wash water is recirculated, so that it is reused in the washer of an earlier stage in the bleachery. Because several acidic and alkaline stages are usually found in the bleaching sequence, the recirculated water is usually of two types: acidic and alkaline. The rejected wash water from the first two stages (usually the chlorination and first extraction stages) of the bleachery is usually combined to give a total bleachery effluent. In recent years there has been intense pressure to reuse a substantial portion of the acidic and alkaline wash waters and this has resulted in a very substantial build-up of the dissolved and suspended wood resin in process liquids. The result of this is usually deposition of wood resin and other substances on the walls of the process equipment. The deposits can subsequently break away from surfaces and cause contamination of the product with pitch-dirt specks. The formation of deposits can be exacerbated by a number of factors in the operation of a kraft mill and these have been well documented (L. H. Allen, Pulp Paper Can., 89(10): T342 (1988)).
In mechanical pulp mills, the wood is converted into wood pulp by mechanical means, usually without chemicals. This results in a physical redistribution of the wood resin similar to that described previously for kraft pulps. Because mechanical pulps are rarely washed, the resin loading in this kind of pulp is often high and can cause deposition problems during subsequent paper manufacture. More intensive reuse of both mechanical pulp, mill and paper machine white waters causes a build-up in the concentration of water-borne resin (L. H. Allen and C. J. Maine, Pulp Paper Can., 79(4): T152 (1978)) and exacerbates the tendency for pitch deposition (L. H. Allen, Trans. Tech. Sect. CPPA, 3(2): 32 (1977)).
Many of the chemicals used to combat foam in pulp and paper mills end up dispersed in the aqueous phase of a pulp suspension. Unfortunately, these often tend to co-deposit with wood resin (G. M. Dorris, M. Douek, and L. H. Allen, J. Pulp Paper Sci., 11(5):J149 (1985); N. Dunlop-Jones and L. H. Allen, J. Pulp Paper Sci., 15(6): J235 (1989)).
It is also an accepted fact that the presence of high amounts of dissolved and dispersed resin in paper machine process liquids usually leads to reduced paper strength and runnability (Wearing, J. T., Ouchi, M. D. Mortimer, R. D., Kovacs, T. G., and Wong, A., J. Pulp Paper Sci., 10(6):J178 (1984)).
Prior procedures for reducing the concentrations of dissolved and dispersed resins in pulp and paper mill process liquids have consisted of one or more of the following approaches:
1. By constantly discarding a fraction of the process liquid, the dispersed resin is bled from the system. This practice is no longer tolerated to the degree that it has been in the past, for environmental reasons.
2. Washing of a wet pulp with water and discarding the wash water is an obvious way to reduce the water-borne resin concentration in a pulp (L. H. Allen and C. J. Maine, Pulp Paper Can., 79(4): T152 (1978)). Nowadays, a substantial fraction of this wash water must be reused, for environmental reasons.
3. The use of a Frotapulper (trade mark of MoDo), followed by a caustic extraction and subsequent treatment of the extraction filtrate by flotation has recently been described for the deresination of sulfite pulps. These treatments are relatively expensive and certainly do not remove all of the resin.
4. Flotation in a flotation save-all is another potential method of removing resin from process liquids. This is an expensive process under most circumstances.
5. The flocculation of dispersed resin with polymers, alum, and flocculants has been used in many paper mills to reduce the concentration of dispersed resin. The cost of the chemical additives for this kind of treatment is appreciable.
6. The addition of talc has been advocated as a way to remove dispersed resin. The resin particles adsorb on the surfaces of talc and are less apt to deposit (L. H. Allen, Trans. Tech. Sect. CPPA, 3(2):32 (1977)). It has, however, recently been demonstrated that only a small fraction of the water-borne resin adsorbs on talc, leaving the balance dispersed in the process liquid. Talc prevents deposition, by contrast, by a detackification mechanism (M. Douek and L. H. Allen, J. Pulp Paper Sci., 17(5): J171 (1991); L. H. Allen and M. Douek, J. Pulp Paper Sci. 19(3): J131-J136 (1993)). Hence, the addition of talc is not a particularly efficient way to remove resin from pulp and paper mill process liquids.
7. The use of dispersants is common for the removal of resin from kraft pulps in the alkaline stages of bleaching. This is a proven way of removing resin from the fibers; however, the resin passes into the dispersed phase, so that there is actually a buildup in the process liquid. Therefore, this is not a method for deresinating the process liquid.